1. Field of the Invention:
The present invention relates to an AC input free power source for use in an electronic apparatus or the like.
2. Description of the Prior Art
A conventional AC input free power source generally comprises, as shown in FIG. 5, a power transformer PT3, a rectifier BD2, a transistor Q6 for protecting an IC (integrated circuit), a switching transistor Q7, a zener diode ZD3 for protecting the switching transistor Q7, a smoothing capacitor C3, a resistor R12 for protecting the transistor Q6, a resistor R13 for limiting the start current of the transistor Q6, and another resistor R14 for controlling the drive current of the transistor Q6. A switching control switch (TRE) for the switching transistor Q7 is fed from the IC to a terminal 7, while a base potential control signal (R) for the transistor Q6 is fed from the IC to a terminal 9. A reference potential (GND) from a terminal 8 and a switching output voltage (VNG) from a terminal 10 are obtained respectively, and a dummy load R15 is connected to terminals 8a and 10a which correspond respectively to the aforesaid terminals 8 and 10.
In the known AC input free power source of such circuit configuration, the secondary output voltage of the power transformer PT3 naturally rises in accordance with an increase of the AC input voltage applied thereto. The input voltage to the AC input free power source ranges from rated 110 volts minus 15% to rated 240 volts plue 15%. The power transformer PT3 is so designed that a required minimum switching output voltage is obtained across the dummy load R15 when the AC input voltage is minimum. In the case when the AC input voltage is at a maximum value, the output voltage becomes about three times the minimum value. The number of on-off actions or the value of the switching frequency of the switching transistor Q7 is determined substantially by the aforementioned power transformer PT3, smoothing capacitor C3 and dummy load R15. It follows, therefore, that the switching frequency of the switching transistor Q7 varies in accordance with the AC input voltage applied to the power transformer PT3. The waveform of the rectified secondary voltage has a low peak value as shown in FIG. 6(a) when the AC input voltage is low, or has a high peak value as shown in FIG. 6(b) when the AC input voltage is high. Accordingly, in the case when the AC input voltage is high, the switching output voltage becomes a switching-off detection level with the capacitor C3 not being fully charged. Therefore the capacitor C3 is discharged in a short period of time, and the switching output voltage becomes a switching-on detection level. Thus, as shown in FIG. 6(d), the number of on-off switching actions repeated per unit time increases to be more than the number obtained in FIG. 6(c) where the AC input voltage is now. Denoted by A and B in FIG. 6(c) and (d) are the charge and discharge voltages of the capacitor C3.
Since the switching action is accompanied with an output loss, it is natural that the power loss increases in accordance with the rise of the switching frequency. Further, due to the high AC input voltage, an increase in the peak current from the power transformer PT3, occurs so that the power transformer PT3, the switching transistor Q7 and so forth are heated. For averting such a state, a great capacity is required to consequently bring about some problems including larger dimensions of the apparatus and a higher cost of its production.